Seal for high-frequency transmission lines



Dec. 21, 1948. D. L. sNow ET AL 2,456,653

SEAL FOR HIGH-FREQUENCY TRANSMISSION LINES Filed D60. ,10, 1942 INVENTQRS, D. L- SNOW.

w. w. HANSEN;

Patented Dec. 21, 1948 SEAL FOR HIGH-FREQUENCY TRANSMISSION LINES Donald L. Snow, Hempstead, and William w. Hansen, Garden City, N. Y., assignors to The Sperry Corporation, a

corporation of Delaware Application December 10, 1942, Serial No. 468,603

3 Claims.

This invention relates to "seals and methods of making them and is particularly concerned with the making of eflicient seals in electronic and like devices. E In "its preferred.embodiment:andmraortant: ;phase pfftheiinventionwill 'be described-with re-;

' gard to the. i'ormation .of anefllcient insulating seal between the inner. and outer conductors 'of a concentric transmission line.

- In electronic apparatus we prefer to make both inner and outer conductors ofan ultra high fre- 'quency concentric transmission line of a cobalt nickel-iron alloy of the type known variously by 'such'trade names as Kovarand 'Fernico. This alloy is used because it has approximately the same-low, coeflicient of thermal expansion as 'borosilicate glass for normal operating temperatures and therefore appears especially adapted for use with insulating glass seals. For the composition of such an alloy, see U. S. Letters Patent No. 1,942,260.

The use of such alloy conductors has, however, raised an unexpected problem because the alloy is a relatively poor electrical conductor, especially for ultra high frequency currents. Thus, although gas-tight seals may readily be provided between such alloy conductors and glass, such a transmission line construction has not proved to be electrically eflicient, especially at high frequencies, and its losses may usually be relatively large.

We have discovered that if the current conducting surfaces of the conductors are electroplated, or similarly coated, with a layer of copper or some other excellent electrical conductor, such greatly increases the electrical efiiciency of the transmission line and reduces the losses therein. This is an important part of the invention. But considerable difflculty has often been encountered in embodying the glass seals in such plated-alloy conductor transmission lines. Apparently as a result of the relatively high temperatures necessary for the sealing operations, severe mechanical defects in the seal have been encountered. These defects usually comprise bubbles in the glass, and pockets between either the plating and the glass or the plating and the alloy surface. Often the copper plating is entirely removed over large sections, thus destroying the advantage of electrical conductivity for which it was employed,

and even more often the glass itself is cracked or becomes easily broken, thereby destroying the vacuum and the insulating qualities of the seal. According to our observation, these bubbles and pockets are caused by gaseous substances released from the metal during the heat of the sealing operation. Very probably these gaseous sub stances come mainly from the copper plating, wherein they are probably absorbed or otherwise contained until released by the sealing heat. A

further important lot ons-invention; as will be described below; includes methods [or preventing 'bubblin'gnnd similar defects g "in the, seal. According "operations of' soldering members made of the v above considered cobalt-nickel-iron alloy to other .metal parts. These joints must be mechanically strong and vacuum tight. Thefeatures of metalto-metal seal are particularlyclaimed in copcnding application SIN. 54,872, filedOctober 16, 1948,

entitled Metal-to-metalseals.

In securing such alloy members'together or to .another metal we preferably employ'a soldering operation, using asolder having an appreciable silver content and a relatively high melting point.

We have found that this silver solder has an extremely high affinity for the nickel-cobalt-iron alloy, and rapidly enters into combination-With it. This high aiiinity of silver solder for the alloy apparently results in an alloying action between the nickel-cobalt-iron alloy and the silver in the solder, whereby the alloy member develops a relatively large newly. alloyed and relatively brittle weakened section which may crack to destroy the vacuum when subjected to temperature variations and other stresses during normal operation.

According to another important phase of the invention, we have developed new methods for silver soldering a member made of nickel-cobaltiron alloy wherebythe mechanical strength of the member is not impaired by the I soldering operation.

With the above in mind it is a major object of the invention to provide a novel method of connecting a metal member in gas tight relation with another metal member and a glassbody.

It is a further object of the invention to provide a high frequency device having eiiicient durable novel sealed Joints which are resistant to the thermal and mechanical stresses encountered during normal operation of the device.

It is a further object of this invention to provide a novel method of sealing glass to metal wherein imperfections in the seal are completely avoided.

A further object of the invention is to provide a novel method of sealing glass to metal wherein absorbed gaseous or gas forming substances are removed from the metal before assembly with the glass. Preferably this removal is effected by a vacuum firing operation. v

A further object of the invention is to provide a novel method of sealing glass to a plated nickele cobalt-iron alloy member wherein imperfections in the seal and the resultant novel productia re completely avoided.

A further object of the invention is to provide a novel method of sealing glass to a copper-plated nickel-cobalt-iron alloy element wherein submisnomer phase ofthe' invention we' haye solved problems "encountered during the stances absorbed or otherwise contained on or in the, plating which may liberate to form bubbles in the seal upon the application of sealing heat are removed prior to incorporation of the element with the class.

A further object of the invention is to provide a novel high frequency energy transmission line and method of making the same.

A further object of the invention is to provide a novel joint between two metal surfaces connected by a solder containing a metal which is readily alloyable with one or both of said surfaces, wherein formation of the alloy is avoided, and methods of making such a joint.

A further object of the invention is to provide a novel joint between two metal surfaces connected by a solder containing a metal which is readily alloyable with at least one of said surfaces, wherein formation of the alloy is avoided by first coating said one surface with a metal which prevents the solder from contacting said one surface.

A further object of the invention is to provide a novel method of silver soldering a nickelcobalt-iron alloy wherein the alloy surface is coated before soldering with a layer of nickel or an equivalent protective coat.

Further objects of the invention will presently appear as the description proceeds in connection with the appended claims and the annexed drawings wherein:

Fig. 1 is a sectional view illustrating an electron tube embodying the invention.

Fig. 2 is an enlarged section of the Kovar-glass joint in the transmission line of Fig. 1.

Fig. 3 is an enlarged section illustrating the soldered metal to metal Joint where the transmission line of Fig. l enters the resonator.

Fig. 4 is another enlarged section illustrating the soldered metal to metal joint between the tube barrel and the resonator.

Fig. 1 illustrates the invention as applied to a tunable high frequency electronic tube of the type employing a hollow resonator and a concentric transmission line.

The hollow resonator I i comprises a cylindrical metal barrel i2 and opposite end walls I: and I4 provided with reentrant poles II and i6 supporting centered and aligned grids i1 and ID. A suitable cathode i8 is located within pole i8 near grid ll.

Wall I4 is an annularly crimped sheet metal member of beryllium copper or a like resilient fatigue resistant material having sufficient stiffness to normally maintain its shape but bein contr'ollably deformable for tuning. Wall i4 is secured permanently, preferably by soldering, to barrel l2 and pole II.

A metal plate 2| is rigid with pole i8 below wall I and parallel to wall ll. Wall i3 and plate 2i, which may be of steel, copper or any suitable metal, are interconnected by the usual springs (only one shown) urging them together and by adjustable tuning screws such as 22. As screws 22 are rotated, the distance between grids i1 and II is changed to tune the hollow resonator. Plate 2| is carried by the usual base (not shown).

The wall of barrel i2 is apertured at 21 to receive a hollow tube 24 comprising the outer conductor of a concentric transmission line. The inner conductor 2' is reversely bent within the resonator to provide an antenna loop 26, and is anchored, as by soldering, to the inner periphery of outer conductor 24. Conductors 24 and 25 are preferably made of a nickel-cobalt-iron alloy such as Kovar, Fernico, or the like.

Conductor 2! is a relatively stiff wire held firmly centrally in the transmission line assembly by an annular seal 21 of glass or some other rigid highly insulating material, which also forms a gas tight seal for the conductor.

Wall II is formed with an annular groove 2| in which is seated and soldered one end of a cylindrical barrel 2!. preferably of Kovar or its above mentioned equivalents. The other end of barrel 29 is enclosed by a glass envelope l0 sealed thereto along rim I so that the interior of the tube is gas tight for evacuation to operating condition.

The above discussed problems of making efficient gas tight joints capable of withstanding the thermal and other mechanical strains attendant to operation of such ultra high frequency apparatus have chiefly been solved by the invention, as embodied in the above, and the solutions will be explained in detail below.

Fig. 2 illustrates a greatly enlarged section through the glass-to-metal seal 21 in the concentric transmission line of Fig. 1.

Referring to the drawings, the central conductor rod or wire 2! of nickel-cobalt-iron alloy is suitably coated with a copper plating 32. The surface of copper plating I2 is heat sealed to the body of glass 21, which in a specific embodiment successfully tested and used is a borosilicate glass known in the trade as G705AJ glass, made by the Corning Glass Company. The outer annular Kovar conductor 24 is internally copper-plated at 33 and also heat-sealed to glass body 21.

According to our invention we have discovered that perfect seals can be almost universally obtained between the conductor and glass elements by the following process.

First, the surface of each conductor to be sealed to the glass is heated, as by hydrogen firing, until its oxide-coated surface is reduced. This is a cleaning operation which makes the conductor surface especially receptive to soldering, and results in an improved solder bond between the plating and Kovar. The next operation comprises electroplating the cleaned conductor surface with copper or an equivalent conductor. Preferably the copperplating is about 0.001 inch thick. It is not, however, material to the invention to plate the Kovar in this exact manner. Any equivalent plating step may be employed.

The next operation is to hydrogen fire the plated copper surface for reducing the oxide there and cleaning the copper-plated surface for sealing with the glass. By hydrogen firing we mean heating in an atmosphere of hydrogen. In this step, we probably remove all gases trapped or otherwise present in the metal and replace the same by hydrogen. As will appear, this step may be eliminated, although it is preferable for practical purposes,

If the glass sealing operation were attempted at this point, it would be subject to the same difficulties as explained above. Hence we have found further treatment needed for insuring good seals.

The final and most important step before sealing is to now heat the copper-plated conductors in a vacuum furnace, preferably maintained at about 900 C. for a duration of approximately five minutes. This heating period is not especially critical but it must be sufficiently long so that the operation releases or removes from the metal, especially from the copper-plated surface, molecules of hydrogen which had been absorbed before and during the preliminary firing operations. For example, the operation has been carhardened assess:

ried out successfully at 4W0. This vacuum heating operation also removes from the metal surface any other foreign substances which, like hydrogen, may become liberated during the ensuing glass-to-metal sealing step and form bubbles or like structural defects in the seal.

After the vacuum ilrlng step, the metal conductor and glass elements are assembled as shown, and heat is supplied to the glass to seal them together. Careful observation shows that no imperfections form in the seal during this operation, which results in perfect glass-to-metal seals.

Althoughthe invention has been described by way of example for sealing copper-plated Kovar to borosilicate glass, it will be understood that it is applicable to the formation of perfect seals between substantially any equivalent metal and glass combinations. For example, the conductors may comprise any metal or alloy having about the same coeflicient of thermal expansion as any arbitrarily chosen glass suitable for the seal glass, and surface platings "and 88 may be of any I suitable metal having the property of being highly electrically conductive and of being attached to glass.

The essential operation of this phase of the invention is the removal of the released gaseous substances. The invention is therefore of sumcient scope to include application of this operation to conductors plated or coated in any manner, and independently of whether they are prepared by the above-outlined preferred process steps.

We have thus provided an electrically eillcient,

mechanically sound and vacuum tight seal for a concentric transmission line. The plated Kovar conductors are Just as electrically eiilcient as would be solid copper conductors, and at the same time the low thermal expansion and other structural advantages of Kovar are retained to keep the seal intact during operating conditions.

While the chief importance of our seal is in ultra high frequency conductor structure, it is very useful for lower frequency applications as well. 1

Fig. 8 is an exaggerated sectional representation of the metal to metal joint between the Kovar tube barrel II and wall I 8. The lower edge and periphery of collar 28 are coated, as by distillation or electrodeposition, with a layer of nickel 88. The nickel-plated end of collar fl is inserted and silver soldered in the usual manner in groove 28, the hardened solder body being indicated at 88 in Pig. 8.

This above soldered joint is sound and firm, and the barrel does not fracture under repeated temperature stress. The nickel coating does not rapidly alloy with the silver in the solder, and prevents the silver from reaching the Kovar. It is obvious that any suitable metal other than nickel may be used to plate the Kovar. It is 8- sential, however, that such metal be capable of forming a good solder bond and also serve as a protective shield for preventing the silver solder from alloying with the Kovar. The metal plating should, of course. have a higher melting point than the solder.

Fig. 4 is an exaggerated sectional illustration of the manner in which Kovar conductor 24 is mounted on barrel i2. Prior to insertion into aperture 28, the associated end of conductor 28 v is coated on its external periphery, preferably by electrodeposition, with a layer 81 of nickel. The plated end of conductor 24 is thrust into aperture is and red in the usual manner, the betas indicated at u.

The phase of the invention related to Figs. 8 and 4 is of especial application and advantage where the soldered parts are relatively large or subjected to relatively heavy stresses.

Since many changes could be made in the above construction and many apparently widely diflerent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In combination, a nickel-cobalt-iron alloy tube, a degasifled coating of copper on the inner surface of said tube, a nickel-cobalt-iron alloy wire having a degasifled coating of copper and positioned coaxially within said tube, and a borosilicate glass cylinder positioned on said wire within said tube and sealed to said copper coated surfaces whereby a vacuum-tight, mechanically rigid coaxial line assembly is provided.

2. In an eiilcient high frequency transmission line, two conductors made or niclsel-cobait-lron alloy, a sealing bodyof glass between said canductors, said alloy having the same low thermal coefllcient of thermal expansion as said glass, and a degasiiled coating of copper between each said conductors and the glass body.

8. A vacuum tube coupling for eflicient conduction of ultra high frequency energy comprising a degaslfled internally copper-plated cylinder of nickel-cobalt-iron alloy adapted to be bonded at one end to a wall of a vacuum tube, a degasiiied copper-plated nickel-cobalt-iron alloy wire positioned coaxially within said cylinder and extending beyond said one end of said cylinder, and a borcsilicate glass element sealed to the copperplated surfaces of said cylinder and said wire whereby said coupling is rendered gas tight and said wire and cylinder are rigidly spaced.

DONALD L. SNOW. WILLIAM W. HANSEN.

REFERENCES CITED The following references are of record in the iile of this patent: UNITED STATES PATENTS Number Name Date 788,518 Whitney May 8, 1808 1,488,110 Mackay May 22, 1828 1,488,808 Fink June 24, 1824 1,847,885 Hoyt July 28, 1828 1,888,878 Robinson Aug. 18, 1827 1,878,818 Fowle Aug. 7, 1928 1,740,448 'Donat Dec. 24, 1828 1,888,288 Iredell Jan. 8, 1888 2,087,482 Scott Oct. 18, 1888 2,082,888 Scott Dec. 1, 1888 2,118,808 Stewart June 7, 1888 2,121,888 Kerschbaum et al. June 21, 1888 2,188,482 Vatter Oct. 18, 1888 2,188,410 Pulfrich et al. June 20, 1888 2,188,870 Ronci Aug. 18, 1888 2,172,878 Kirch Sept. 12, 1888 2,278,188 Ohnesorge ..s--- Feb. 17, 1842 2,278,488 Holman Feb. 17, 1842 2,884,888 Mouromtseif et al. Feb. 8, 1846 FOREIGN PATENTS Number Country Date 248,084 Great Britain July 1, 1888 

